End-to-end Calibration of NEXRAD Differential Reflectivity with Metal Spheres

This study is concerned with the absolute calibration of radar differential reflectivity, ZDR. By definition, ZDR = 10 log (ZH/ZV), where ZH and ZV are the reflectivity values at horizontal and vertical polarization, respectively. The name of the calibration game is symmetry between horizontal and vertical channels, and the accurate matching of all radar quantities that contribute to asymmetry (antenna gain, transmitted power, received power). A metal sphere is a 0 dB target for ZDR at a level of accuracy exceeding any expectation. The generally desired level of accuracy for ZDR is 0.1 dB (Sachidananda and Zrnic, 1987; Zrnic et al., 2010) and has come to be known as the “Holy Grail” in polarimetric radar work. Now that the NEXRAD radars in the US have been upgraded with dual pol capability, the 0.1 dB criterion is increasingly recognized as a formidable challenge to all calibration methods presently available.